Hot-dip galvanized steel sheet and manufacturing method thereof

Abstract

A hot-dip galvanized steel sheet includes: a steel sheet; and a plated layer on a surface of the steel sheet, a microstructure contains, by volume fraction, equal to or more than 20% and equal to or less than 99% in total of one or two of martensite and bainite, a residual structure contains one or two of ferrite, residual austenite of less than 8% by volume fraction, and pearlite of equal to or less than 10% by volume fraction, tensile strength is equal to or greater than 980 MPa, the plated layer is a hot-dip galvanized layer which contains oxides including one or two or more of Si, Mn, and Al, contains equal to or less than 15 mass % of Fe, and a remainder including Zn, Al, and unavoidable impurities, and when a cross section including the steel sheet and the hot-dip galvanized layer is seen in a sheet thickness direction, a projected area ratio is equal to or more than 10% and equal to or less than 90%.

The invention claimed is: 
     
       1. A hot-dip galvanized steel sheet comprising:
 a steel sheet; and 
 a plated layer on a surface of the steel sheet, 
 wherein the steel sheet contains, by mass %, 
 C: equal to or more than 0.05% and less than 0.40%, 
 Si: 0.5% to 3.0%, 
 Mn: 1.5% to 3.0%, 
 O: limited to 0.006% or less, 
 P: limited to 0.4% or less, 
 S: limited to 0.01% or less, 
 Al: limited to 2.0% or less, 
 N: limited to 0.01% or less, and 
 the remainder including Fe and unavoidable impurities, 
 wherein a microstructure of the steel sheet contains, 
 by volume fraction, equal to or more than 20% and equal to or less than 99% in total of one or two of a martensite and a bainite, and 
 a residual structure including a ferrite, and one or two of a residual austenite of less than 8% by volume fraction, and a pearlite of equal to or less than 10% by volume fraction, 
 wherein a tensile strength of the steel sheet is equal to or greater than 980 MPa, 
 wherein the plated layer is a hot-dip galvanized layer which contains oxides including one or two or more of Si, Mn, and Al, contains equal to or less than 15 mass % of Fe, and the remainder including Zn, Al, and unavoidable impurities, and 
 wherein when a cross section including the steel sheet and the hot-dip galvanized layer is seen in a sheet thickness direction, a projected area ratio which is an area ratio obtained by dividing a length of the oxides projected to an interface between the hot-dip galvanized layer and the steel sheet by a length of the interface between the hot-dip galvanized layer and the steel sheet, is equal to or more than 19% and equal to or less than 62%. 
 
     
     
       2. A galvannealed steel sheet comprising:
 a steel sheet; and 
 a plated layer on a surface of the steel sheet, 
 wherein the steel sheet contains, by mass %, 
 C: equal to or more than 0.05% and less than 0.40%, 
 Si: 0.5% to 3.0%, 
 Mn: 1.5% to 3.0%, 
 O: limited to 0.006% or less, 
 P: limited to 0.04% or less, 
 S: limited to 0.01% or less, 
 Al: limited to 2.0% or less, 
 N: limited to 0.01% or less, and 
 the remainder including Fe and unavoidable impurities, 
 wherein a microstructure of the steel sheet contains, 
 by volume fraction, equal to or more than 20% and equal to or less than 99% in total of one or two of a martensite and a bainite, and 
 a residual structure including a ferrite, and one or two of a residual austenite of less than 8% by volume fraction, and a pearlite of equal to or less than 10% by volume fraction, 
 wherein a tensile strength of the steel sheet is equal to or greater than 980 MPa, 
 wherein the plated layer is a galvannealed layer which contains oxides including one or two or more of Si, Mn, and Al, contains equal to or more than 7 mass % and equal to or less than 15 mass % of Fe, and the remainder including Zn, Al, and unavoidable impurities, and 
 wherein when a cross section including the steel sheet and the galvannealed layer is seen in a sheet thickness direction, a projected area ratio which is an area ratio obtained by dividing a length of the oxides projected to an interface between the galvannealed layer and the steel sheet by a length of the interface between the galvannealed layer and the steel sheet, is equal to or more than 19% and equal to or less than 62%. 
 
     
     
       3. The hot-dip galvanized steel sheet according to  claim 1 ,
 wherein the microstructure contains, by volume fraction, 40% to 80% of ferrite. 
 
     
     
       4. The hot-dip galvanized steel sheet according to  claim 1 ,
 wherein the microstructure contains, by volume fraction, more than 60% and equal to or less than 99% of one or two of martensite and bainite. 
 
     
     
       5. The hot-dip galvanized steel sheet according to  claim 1 ,
 wherein the steel sheet further contains, by mass %, one or two or more of 
 Cr: 0.05% to 1.0%, 
 Mo: 0.01% to 1.0%, 
 Ni: 0.05% to 1.0%, 
 Cu: 0.05% to 1.0%, 
 Nb: 0.005% to 0.3%, 
 Ti: 0.005% to 0.3%, 
 V: 0.005% to 0.5%, 
 B: 0.0001% to 0.01%, 
 Ca: 0.0005% to 0.04%, 
 Mg: 0.0005% to 0.04%, 
 REM: 0.0005% to 0.04%. 
 
     
     
       6. A manufacturing method of the hot-dip galvanized steel sheet of  claim 1 , the method comprising:
 casting a molten steel having the composition of the steel sheet to obtain a steel; 
 heating the steel to a first temperature range of 1100° C. to lower than 1300° C., directly or after cooling once; 
 completing a hot rolling of the steel at a temperature equal to or higher than an Ar3 transformation point; 
 coiling the steel in a second temperature range of 300° C. to 700° C.; 
 pickling the steel; 
 performing cold rolling of the steel with a cumulative rolling reduction of 40% to 80% using a cold rolling mill including a work roll having a roll diameter of 200 mm to 1400 mm; 
 retaining the steel in a third temperature range of 550° C. to 750° C. for 20 seconds to 2000 seconds during heating the steel to an annealing temperature, when the steel passes through a continuous galvanizing line; 
 maintaining the steel in a fourth temperature range of 750° C. to 900° C. for 10 seconds to 1000 seconds, in an N 2  atmosphere in which an H 2  concentration is equal to or less than 20% and a dew point is equal to or higher than −20° C., while performing an annealing; 
 performing a first cooling of cooling the steel to a fifth temperature range of 500° C. to 750° C. at an average cooling rate of 1° C./sec to 200° C./sec; 
 performing a second cooling of cooling the steel to a sixth temperature range between a temperature which is lower than a hot dip galvanizing bath temperature by 40° C. and a temperature which is higher than the hot dip galvanizing bath temperature by 50° C., at an average cooling rate which is 1° C./sec to 200° C./sec and is faster than the average cooling rate of the first cooling; 
 galvanizing the steel by immersing the steel in a hot dip galvanizing bath which flows at a flow velocity of 10 m/min to 50 m/min after setting a plating bath immersion sheet temperature which is a temperature when immersing the steel in the hot dip galvanizing bath, as the sixth temperature range; and 
 cooling the steel to a temperature equal to or lower than 40° C. 
 
     
     
       7. A manufacturing method of the galvannealed steel sheet of  claim 2 , the method comprising:
 casting a molten steel having the composition of the steel sheet to obtain a steel; 
 heating the steel to a seventh temperature range of 1100° C. to lower than 1300° C., directly or after cooling once; 
 completing a hot rolling of the steel at a temperature equal to or higher than an Ar3 transformation point; 
 coiling the steel in an eighth temperature range of 300° C. to 700° C.; 
 pickling the steel; 
 performing cold rolling of the steel with a cumulative rolling reduction of 40% to 80% using a cold rolling mill including a work roll having a roll diameter of 200 mm to 1400 mm; 
 retaining the steel in a ninth temperature range of 550° C. to 750° C. for 20 seconds to 2000 seconds during heating the steel to an annealing temperature, when the steel passes through a continuous galvanizing line; 
 maintaining the steel in a tenth temperature range of 750° C. to 900° C. for 10 seconds to 1000 seconds, in an N 2  atmosphere in which an H 2  concentration is equal to or less than 20% and a dew point is equal to or higher than −20° C., while performing an annealing; 
 performing a third cooling of cooling the steel to an eleventh temperature range of 500° C. to 750° C. at an average cooling rate of 1° C./sec to 200° C./sec; 
 performing a fourth cooling of cooling the steel to a twelfth temperature range of 500° C. to 25° C., at an average cooling rate which is 1° C./sec to 200° C./sec and is faster than the average cooling rate of the third cooling; 
 heating the steel again to a thirteenth temperature range of 350° C. to 500° C., in a case where a cooling stop temperature of the fourth cooling is lower than 350° C.; 
 retaining the steel in the thirteenth temperature range; 
 galvanizing the steel by immersing the steel in a hot dip galvanizing bath which flows at a flow velocity of 10 m/min to 50 m/min after setting a plating bath immersion sheet temperature which is a temperature when immersing the steel in the hot dip galvanizing bath, as a fourteenth temperature range between a temperature which is lower than a hot dip galvanizing bath temperature by 40° C. and a temperature which is higher than the hot dip galvanizing bath temperature by 50° C.; 
 performing an alloying treatment to the steel at a fifteenth temperature range of equal to or lower than 600° C.; and 
 cooling the steel to a temperature equal to or lower than 40° C. 
 
     
     
       8. The manufacturing method of the hot-dip galvanized steel sheet according to  claim 6 , wherein the annealing is performed at a temperature lower than 840° C. 
     
     
       9. The manufacturing method of the hot-dip galvanized steel sheet according to  claim 6 , wherein the annealing is performed at a temperature equal to or higher than 840° C. 
     
     
       10. The manufacturing method of the hot-dip galvanized steel sheet according to  claim 6 ,
 wherein the molten steel further contains, by mass %, one or two or more of 
 Cr: 0.05% to 1.0%, 
 Mo: 0.01% to 1.0%, 
 Ni: 0.05% to 10%, 
 Cu: 0.05% to 1.0%, 
 Nb: 0.005% to 0.3%, 
 Ti: 0.005% to 0.3%, 
 V: 0.005% to 0.5%, 
 B: 0.0001% to 0.01%, 
 Ca: 0.0005% to 0.04%, 
 Mg: 0.0005% to 0.04%, and 
 REM: 0.0005% to 0.04%. 
 
     
     
       11. The galvannealed steel sheet according to  claim 2 ,
 wherein the microstructure contains, by volume fraction, 40% to 80% of ferrite. 
 
     
     
       12. The galvannealed steel sheet according to  claim 2 ,
 wherein the microstructure contains, by volume fraction, more than 60% and equal to or less than 99% of one or two of martensite and bainite. 
 
     
     
       13. The galvannealed steel sheet according to  claim 2 , wherein the steel sheet further contains, by mass %, one or two or more of:
 Cr: 0.05% to 1.0%, 
 Mo: 0.01% to 1.0%, 
 Ni: 0.05% to 1.0%, 
 Cu: 0.05% to 1.0%, 
 Nb: 0.005% to 0.3%, 
 Ti: 0.005% to 0.3%, 
 V: 0.005% to 0.5%, 
 B: 0.0001% to 0.01%, 
 Ca: 0.0005% to 0.04%, 
 Mg: 0.0005% to 0.04%, and 
 REM: 0.0005% to 0.04%. 
 
     
     
       14. The manufacturing method of the galvannealed steel sheet according to  claim 7 , wherein the annealing is performed at a temperature lower than 840° C. 
     
     
       15. The manufacturing method of the galvannealed steel sheet according to  claim 7 , wherein the annealing is performed at a temperature equal to or higher than 840° C. 
     
     
       16. The manufacturing method of the galvannealed steel sheet according to  claim 7 , wherein the molten steel further contains, by mass %, one or two or more of:
 Cr: 0.05% to 1.0%, 
 Mo: 0.01% to 1.0%, 
 Ni: 0.05% to 1.0%, 
 Cu: 0.05% to 1.0%, 
 Nb: 0.005% to 0.3%, 
 Ti: 0.005% to 0.3%, 
 V: 0.005% to 0.5%, 
 B: 0.0001% to 0.01%, 
 Ca: 0.0005% to 0.04%, 
 Mg: 0.0005% to 0.04%, and 
 REM: 0.0005% to 0.04%.